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bulletin: museum of comparative zoology. 



3. Another way in which the mass changes form without growth is 

 by vacuolization. Through vacuolization there arise, e. g\, the blastula in 

 many eggs, the secondary body cavitj' in many Invertebrates (K. & H., 

 Fig. 131, 689), and in Bryozoa the cavities of the bud. This process of 

 vacuolization, which I introduce for convenience at this place, is not 

 confined to masses. Many cords by vacuolization (canalization) become 

 tubules, and many layers become divided into two. Of vacuolated cords, 

 I need refer only to the formation of capillaries in Vertebrates and of 

 nephridia in Invertebrates ; of vacuolated layers, to the origin of the 

 ccelom in most Vertebrates. In all cases, the acquisition of a mass 

 of water at the centre causes a rearrangement of the nuclei. 



4. Perhaps this is the most fitting place to mention the process of 

 Dispersion of elements, which occurs not only in bodies originally 

 formed by aggregations of mesenchymatous elements (Fig. 28), but 

 also in layers having an epithelial origin, e. g. the ectoderm of Dis- 

 tomum. (K. & H., Fig. 88.) 



IV C . The remaining processes occurring in protoplasmic masses are of 



such a nature as to alter the number 

 of masses. We can distinguish, corre- 

 spondingly, two classes : the first in- 

 cluding such as have to do with the 

 formation of two masses from one, 

 through division ; the second including 

 those which have to do with the union 

 of two masses into one. The first, 

 then, are division processes ; the sec- 

 ond, fusion processes. 



5. Under the first head we may in- 

 clude the process of constriction, by 

 which two more or less independent 

 masses arise from one. An illustra- 

 tion of this is found in the case 

 of embryonic fission described for 

 some Bryozoa by Harmer (Figure 29). 

 Closely allied to this is the process of 

 sloughing off of a part of the body in 

 metamorphosis, one of the most striking instances of which is shown in 



Fir,. 29. 



Fig. 29. Section through a brood chamber of Crisea eburnea, showing the con- 

 stricting off of secondary embryos from the primary embryo. After Harmer, '93, 

 Quart. Jour. Mic. Sci, XXXIV., Plate XXIII. Fig. 11. 



